The electrical energy delivered in particular by the catenary to a locomotive needs to be measured in order to enable the supplier to bill consumers. This need is relatively new since it results from recent regulations requiring a plurality of electricity supply operators to be free to compete on a network, and for a plurality of operators to be free to use said energy on the same network.
In principle, implementing such measurement is extremely simple: it suffices to measure the current drawn, its voltage, and the duration of its consumption, either continuously or by sampling at a suitable frequency, so as to take account of variations in those two parameters over time.
In practice, things are much more complicated because the measurement devices are in the environment of a high voltage network (from 1500 volts direct current (DC) to 25,000 volts alternating current (AC) depending on the portions of the network being traveled) when it comes to acquiring the variables, and in a low voltage environment when it comes to generating and processing signals. Those two environments need to be isolated galvanically, which requires particular care and attention.
Known means for providing such galvanic isolation are generally implemented using transformers that are included in the measurement devices, with the level of isolation that is required demanding the use of high quality dielectric materials between the primary and the secondary of such a transformer, and above all requiring a volume that is great enough to keep the conductive parts as far apart as possible from one another.
That volume and that weight need to be located on the roof of the motive power unit. They are in addition to all of the other equipment on the roof of the traction unit such that under the best of circumstances there is an increase in the frontal area of the equipment, thus creating drag that is an impediment for high-speed trains, and more often is quite simply impossible since there is not enough room in this particularly congested space.